1. Field of the Invention
This invention relates to a current injection type oscillation device using a frequency region from the millimeter wave to the terahertz wave (from 30 GHz to 30 THz). More particularly, the present invention relates to a current injection type oscillation device having a resonant tunneling diode structure.
2. Description of the Related Art
Non-destructive sensing techniques using electromagnetic waves in a frequency region from the millimeter wave to the terahertz wave (from 30 GHz to 30 THz) (to be referred to simply as “terahertz wave” hereinafter) have been and are being developed. In the field of application of electromagnetic waves of this frequency band, safe imaging techniques have been and are being developed for see-through inspection apparatus that can replace X-ray inspection apparatus. Furthermore, spectroscopic technologies for looking into the physical properties including the bonded status of a substance through determination of the absorption spectrum and the complex permittivity of the inside of the substance, biomolecule analyzing techniques and techniques for evaluating the carrier concentration and the mobility are on the way of development.
Additionally, inspection apparatus for inspecting the presence or absence of a substance having an absorption spectrum unique to the terahertz band, or so-called fingerprint spectrum, are in the stage of preliminary development. Such an inspection apparatus can operate quickly and efficiently by discretely providing a plurality of oscillators having oscillation frequencies that are close to the fingerprint spectrum of the object substance of inspection (typically from 0.1 THz to 10 THz) because it does not involve any sweep in the time domain or the frequency domain.
THz wave generating unit include units for generating a pulse wave by irradiating a femtosecond laser beam onto a photoconductive device and parametric oscillation units for generating a wave of a specific frequency by irradiating a nanosecond laser beam onto a nonlinear crystal. However, such units are all of the photoexcitation type that is accompanied by limitations in terms of downsizing and reduction of power consumption rate.
Structures produced by using a quantum cascade laser and a resonant tunneling diode (RTD) are being studied as current injection type devices that operate in the terahertz wave region. Particularly, resonant tunneling diode type devices that are disclosed in Japanese Patent Application Laid-Open No. 2007-124250 and Japanese Patent Application Laid-Open No. H08-116074 are expected to operate at room temperature at or near 1 THz. Such a device is typically formed by a quantum well of InGaAs/InAlAs that is made to grow by epitaxial growth by means of a lattice-aligned system on an InP substrate. It illustrates a negative resistance as illustrated in FIG. 12 of the accompanying drawings for the voltage-current (V-I) characteristic and the oscillation characteristic and is observed to be oscillating near this region.
A planar antenna structure formed on the surface of a substrate as disclosed in Japanese Patent Application Laid-Open No. 2007-124250 is suitably employed as a resonance structure for oscillation. A cavity resonator having a three-dimensional structure is formed in the case of the type disclosed in Japanese Patent Application Laid-Open No. H08-116074. It is a structure where the wall surfaces and the rear surface of the device are also covered by an electrode.